Throughout the animal kingdom, the formation of the nervous system involves the elimination of many cells, soon after their generation. This phenomenon, known as naturally occurring cell death, has precise time schedules, is observed in the vast majority of nervous structures and causes the 1oss of 15 - 85% of the neurones generated initially. Elimination of erroneous projections, as well as proper size matching between connecting structures can be achieved through cell death. However if elimination of wrongly projecting cells is prevented experimentally, cell death takes place with its normal time course and in the normal amount. On the contrary manipulations of the target structure many times profoundly affect the amount of cell death in a nervous center, but conflicting data exist as to this issue. It is a common idea that different mechanisms underlie cell death in different cases and a single explanation would be doomed to failure. We present here a general model of trophic interactions which accounts for naturally occurring histogenetic cell death in the nervous system of vertebrates. The model can quantitatively predict the outcome of the experiments tried to investigate cell death and finally reconciles in a single framework experiments until now viewed as contrasting.